PROCEDURES BASICS

Defining Procedures

A procedure in x86 assembly serves the same purpose as a function in higher-level languages. It encapsulates a sequence of instructions to perform a specific task.

In this example, I define a procedure named addTwo. The procedure adds the values stored in the eax and ebx registers and returns the result with ret.

addTwo:
    add eax, ebx
    ret

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Calling Procedures

To call a procedure, I use the call instruction followed by the name of the procedure.

addTwo:
    add eax, ebx
    ret

main:
    mov eax, 2
    mov ebx, 3
    call addTwo

When a procedure is called, the return address is pushed onto the stack. The RET instruction retrieves this return address from the stack, effectively returning control to the caller.

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Debugging the Stack

Let’s use GDB to inspect the stack and understand how return addresses are managed.

• info register esp - Displays the current value of the stack pointer esp;
• x/x $esp - Displays the value at the top of the stack;

Understanding how return addresses are stored on the stack is crucial for computer security. Exploits such as buffer overflows can manipulate return addresses to execute malicious code.

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Example Code

section .text
    global main

addTwo:             ; define a procedure called addTwo
    add eax, ebx    ; add the two arguments
    ret             ; return the result

main:
    mov eax, 4      ; set the first argument to 4
    mov ebx, 1      ; set the second argument to 1
    call addTwo     ; call the addTwo procedure
    mov ebx,eax     ; store the result of eax in ebx
    mov eax, 1      ; set eax to 1
    int 80h         ; exit the program, system call